Participation in after-school programs is an important lever to improve adolescents' health and well-being; however, well-defined measurement of the quality of participation in these programs is limited. The present study validated a newly designed measure of participation in a sample of urban youth enrolled in community-based after-school programs. Exploratory and confirmatory factor analyses were used to test the structure of the 20-item Tiffany-Eckenrode Program Participation Scale (TEPPS). Results suggest that the scale comprises four subscales (Personal Development, Voice/Influence, Safety/Support and Community Engagement). The TEPPS was also correlated with several commonly used measures of program participation. Findings from this article provide support for the use of the newly designed scale as a valid and reliable measure of quality program participation by youth. (Contains 5 tables and 1 footnote.)